The present invention relates to a cement clinker grinding method using a vertical roller mill and a cement clinker grinding apparatus, by which cement clinkers and cement materials are ground and produced as cement products by use of the vertical roller mill.
FIG. 24 shows a prior art vertical roller mill which has been used for producing the cement products by grinding the cement clinkers, the cement materials and so on. In FIG. 24, a vertical roller mill 1 of so-called separator built-in type includes a housing 2, a table 3 arranged in the housing 2 so as to rotate about a vertical axis and a motor 4 for rotating the table 3. A plurality of rollers 5 are pressed on the table 3. In the housing 2, a separator 6 is arranged above the table 3 and the rollers 5. Feed material to be ground, which has been supplied from a material chute 7, is ground between the table 3 and the rollers 5. Air is introduced into a clearance 8 defined between an outer periphery of the table 3 and an inner periphery of the housing 2 in order to blow up the ground material by a resultant air flow 9. The ground material blown up is classified in the separator 6, so that the fine powder as the products is conveyed by air flow through a pipe line 10. As shown with an arrow 11, the coarse powder is returned onto the table 3 for circulation and ground thereon again.
In the above-mentioned vertical roller mill 1 of FIG. 24, the pressure of the roller 5 per full sectional area of roller is established to be less than about 8 kg/cm.sup.2. Such an establishment of the roller pressure allows the resultant products to have a particle size suitable for the cement and makes a basic unit of power satisfactory.
Further more, in the prior art vertical roller mill 1, a circulation ratio of the ground material (alias, a ratio of grinding occasion, that is, a ratio of a flowing amount of the coarse powder returned from the separator 6 onto the table 3 to a flowing amount of feed material charged into the vertical roller mill through the material chute 7) is estimated more than 1000 per cent (%), so that the coarse powder is instantly returned onto the table 3 for grinding it again. Thus, as an example, in order to realize the grinding amount of 100 ton/hour, the ground material over 1000 ton/hour has to be conveyed with air flow.
One problem to be solved in the conventional vertical roller mill 1 of FIG. 24 resides in narrow particle-size distribution of the products obtained through the pipe line 10. Particularly, regarding the operation for grinding the cement clinkers, the particle size distribution of the products has a great influence on the quality of products. In the above-mentioned prior art, when the particle-size distribution varies due to the aged deterioration etc., it is difficult to adjust the particle-size to distribution properly.
Since the conventional vertical roller mill 1 of FIG. 24 has a small production ratio of the fine powder, the resultant products have a great deal of intermediate particles in comparison with the product obtained by a tube mill. Thus, there exists such a tendency that (n) value of Rosin-Rammler diagram is large, thereby producing the products characterized by a narrow particle-size distribution. The cement products having a narrow particle-size distribution causes the quantity of water required for the standard softness, the concrete experiment etc. to be increased, so that the quality of cement products would be deteriorated.
The other problem of the vertical roller mill of FIG. 24 resides in largeness of air-swept, that is, the power consumption of suction fan for air flow conveyance is too large. As mentioned above, in order to convey the ground material to the separator 6 inside the housing 2 and subsequently classify the material therein, the material ground between the table 3 and the rollers 5 has to be blown up by a great flow of air introduced from the underside of the table 3 through the clearance 8. Therefore, the pressure loss within the vertical roller mill 1 is relatively large, and further the power consumption of the suction fan is also very large.
The other problem of the vertical roller mill 1 of FIG. 24 resides in lowered temperature of the products. The vertical roller mill 1 is constructed in a manner that a large amount of cool air is introduced for air flow conveyance of the ground material. Therefore, because of the progressed cooling performance, the temperature of the ground material is lowered. It is noted that, together with the cement clinkers, gypsum is also contained in the material to be ground. By lowering the temperature, this gypsum becomes to be the cement products as it is dihydrate gypsum. Generally, it is desirable that the crystallization water of gypsum is transformed into either hemihydrate gypsum or anhydrate gypsum during the grinding process. As a result, a problem arises in that there exists a possibility of false cement setting due to the dihydrate gypsum. Therefore, in order to prevent the false cement setting, an installation for either introducing hot air from the outside or circulating exhaust gas from the suction fan would be required. In such a case, the installation cost would be increased with an increased capital investment.